Sodium tripolyphosphate (STPP) is prepared from a feedstock containing disodium orthophosphate (disodium hydrogen phosphate) and monosodium orthophosphate (sodium dihydrogen phosphate) in a 2:1 mole ratio. A convenient route for the preparation of sodium orthophosphate mixtures having the required 2:1 mole ratio is the reaction of soda ash with phosphoric acid according to the equation: EQU 5Na.sub.2 CO.sub.3 +6H.sub.3 PO.sub.4 .fwdarw.2NaH.sub.2 PO.sub.4 +4Na.sub.2 HPO.sub.4 +5CO.sub.2 +5H.sub.2 O (I)
It is desirable to prepare mixed monosodium and disodium orthophosphates in a medium containing a minimum of water since excess water must be removed at some expense at the conclusion of the process. "Light soda ash", a synthetic soda ash derived from the Solvay process, is highly reactive with concentrated phosphoric acid in stoichiometric proportions when contacted in a slurry reaction medium containing relatively little water. However, Solvay soda ash is not of assured future supply because of diminishing use of the Solvay process. It is necessary to develop alternate processes for the preparation of orthophosphate mixtures which are not dependent upon Solvay soda ash as a principle reactant.
Monosodium and disodium orthophosphate salt mixtures desirably have very low levels of unreacted soda ash or phosphoric acid. For example, phosphate salts for meat curing (see U.S. Pat. No. 2,513,094 to Hall) desirably have low levels of unreacted carbonate.